Method for stimulating well production

ABSTRACT

The production of hydrocarbons from a subterranean hydrocarbonbearing formation containing acid-soluble components, such as one composed at least in part of dolomite or limestone, is stimulated by injecting into the formation a composition comprising an aqueous solution of a mineral acid having dissolved therein a phosphate ester of prescribed formula. The elimination of plugging of capillary openings within the formation and mineral scale deposition on production equipment due to postprecipitation of dissolved salts subsequent to acidization by means of the said phosphate ester results in a substantial improvement in hydrocarbon recovery.

Unllefl mates rawlu Tate 1 1March 13, 1973 METHOD FOR STIMULATING WELL 3,483,925 12/1969 Slyker .25 2/1155 8 x PRODUCTlQN 3,502,587 3 1970 Stanford et a1 252/855 B x 3,630,285 12/1971 Claytor, Jr. et al ..166/307 1 lnvemorl Jack Tate, Houston, 3,656,551 4 1972 Biles ..166/310 [73] Assignee: Texaco Inc., New York, N.Y.

Primary Examiner-Stephen J. Novosad [22] Filed: June 21,1971 Attorney-Thomas H. Whaley et a1.

21 Appl. No.. 155,312 [57] ABSTRACT The production of hydrocarbons from a subterranean [52] U.S.Cl ..166/307, 166/308 hydmcarbombearing formation Containing acid solu [S 1] Int. Cl ..E21b 43/26, E21b 43/27 ble components, Such as one composed at least in part [58] 'Fleld of Search"166/307 281; 252/855 of dolomite or limestone, is stimulated by injecting 252/855 C into the formation a composition comprising an aqueous solution of a mineral acid having dissolved therein [56,] References C'ted a phosphate ester of prescribed formula. The elimina- UNITED STATES PATENTS tion of plugging of capillary openings within the for- 1 mation and mmeral scale deposition on production 2,802,531 5 8/1957 Cardwell et al ..166/307 X equipment due to post-precipitation of dissolved salts 3,233,672 2/1966 Carpenter ..166/307 subsequent to acidization by means of the said ,319, 1 5/ KnOX 1 phosphate ester results in a substantial improvement 3,348,613 3111... in hydrocarbon recovery 3,429,824 2/1969 Tate... ....252/8.55 B X 3,488,289 1/1970 Tate i ..252/8.55 B X 13 Claims, No Drawings METHOD FOR STIMULATING WELL PRODUCTION FIELD OF THE INVENTION This invention relates to a method for stimulating the production of fluids from earthen formations. More particularly, this invention relates to a method in which the productivity of a hydrocarbon-bearing formation containing acid-soluble components and with or without water-sensitive clays or shales is improved upon treatment of the formation with an aqueous solution of a mineral acid and a phosphate ester of prescribed formula, said phosphate ester effecting the elimination of plugging of capillary openings due to post-precipitation of dissolved salts subsequent to acidization as well as effecting elimination of mineral scale on production equipment such as pumps, tubing and the like caused by such precipitation.

DESCRIPTION OF THE PRIOR ART The technique of increasing the permeability of a subterranean hydrocarbon-bearing formation and of removing obstructing acid-soluble minera'l scale for the purpose of stimulating the production of fluids therefrom has long been practiced in the art.- One such method commonly employed is known as acidizing which is widely utilized in treating subsurface acidsoluble geological formations, e.g., limestone, dolomite, etc. The technique is not limited to application in formations of high acid solubility. Sand-stone and gypsium-containing formations may require acidization if the produced water is unstable with respect to CaCo In the usual well-acidizing procedure, a non-oxidizing mineral acid is introduced into the well and under sufficient pressure is forced into the adjacent subterranean formation where it reacts with formation components and deposited mineral scale particularly the carbonates such as calcium carbonate, magnesium carbonate, etc., to form the respective salt of the acid, carbon dioxide and water. The usual mineral acid employed in such acidization procedures is hydrochloric acid.

During the acidizing process passageways for fluid flow are created or existing passageways therein are enlarged thus stimulating the production of oil, water, brines and various gases. If desired, the acidization may be carried out at an injection pressure sufficiently great to create fractures in the strata or formation which has the desired advantage of opening up passageways into the formation along which the acid can travel to more remote areas from the well bore. The salt formed uponneutralization of the acid is extensively water soluble and is readily removed by reverse flow from the formation via the well bore.

There are, however, troublesome complications attenting the use of hydrochloric acid or other similar non-oxidizing. mineral acids. In the acidizing process, the following primary beneficialreaction occurs: CaCO +2HCl CaCl,+H,O+CO,. Under the higher pressures required to conduct an acidization, the C0,. is dissolved in the reaction mixture consisting of spent acid and connat'e water: CO -F ,0 ugo, *'H +H- CO1, t- The equilibria may be summarized and written: Ca(.HCO CaCO' +I-I,CO H H,O 2.

After acidization is completed, the well is often backflowed in the case of a water injection well (in order to clean out formation and tubing) and put back on production in the case of a producing oil or gas well. In both cases, pressure diminishes, CO breaks out of solution, inducing CaCO to precipitate. Such precipitation, when it occurs within the capillaries of a tight formation or on the tubing or annulus as a mineral scale, can severely lessen production or injection rate by plugging such capillaries or well equipment.

It is known that molecularly dehydrated polyphosphates are effective in retarding CaCO precipitation.

These polyphosphates are unsatisfactory in the method of the present invention because they undergo rapid hydrolysis in the presence of the mineral acid component. As a result, the scale inhibiting properties of these polyphosphates are destroyed. In addition, one hydrolytic reaction product, the phosphate ion (PO can precipitate with calcium or barium ions present in the produced water, causing additional plugging or scale deposition and further aggravating the problem. The so-called glassy phosphates are known scale inhibitors. However, these glassy phosphates are unsatisfactory because of their slight solubility in acidic media and the tendency to form objectionable hydrolytic reaction products.

It is also known to employ various organic polymers to prevent the precipitation of the mineral salts. Many of these polymeric materials are unstable in mineral acids. In such acidic media they undergo spontaneous depolymerization to an ineffective species. A representative polymeric material which undergoes such hydrolysis in the presence of acids is polyacrylamide. In addition, this polymer has a further disadvantage in that it is unstable in aqueous media at temperatures of about 300F. and upwards. Many wells that are being treated by the method of the present invention have bottom hole temperatures of 250-300F or higher.

The chemically altered natural polymers and natural polymers themselves, are effective inhibitors to prevent the precipitation of mineral salts. However, many of these materials such as sodium carboxymethylcellulose precipitate or decompose in the presence of mineral acids. Other known sequestering agents such as'citric or tartaric acids, and/or complexing agents such as ethylenediaminetetraacetic acid and its water-soluble salts are known inhibitors to prevent the deposition of boiler scale in aqueous media. However, such materials are not applicable in the method of the present invention because they are not appreciably surface active and do not adsorb on the formation face.

It is therefore, the principal object of the present invention to overcome the defects of the prior art in acidizing fluid bearing formations such as hydrocarbon-bearing formations, etc., by providing a method of acidization employing the novel composition of this invention.

SUMMARY OF THE INVENTION This invention encompasses and includes a method of increasing the production of fluids from a subtertion under a pressure greater than the formation pressure an aqueous acidizing composition hereinafter more fully described, maintaining said composition in contact with the formation strata for a time sufficient for the acid to chemically react with the acid-soluble components of the formation and/or acid-soluble mineral scale deposited on production equipment to etch or enlarge passageways through the strata and remove the scale and thereby increasing substantially the flow capacity of the said subterranean formation.

The novel aqueous acidizing composition of this invention comprises an aqueous solution of non-oxidizing mineral acid, such as hydrochloric or sulfuric, having dissolved therein a phosphate ester of prescribed formula. The concentration of acid present in the subject composition is such that it is capable of reacting with the acid-soluble components of the fluid-bearing strata.

DETAILED DESCRIPTION OF THE INVENTION In its broadest embodiment the method of the present invention comprises introducing into a subsurface formation containing acid-soluble components an aqueous acid solution of a phosphate ester of prescribed formula wherein the said solution is maintained in contact with the formation for a time sufficient to chemically react with the formation and/or acid soluble mineral scale deposited on production equipment so as to increase substantially the flow capabilities of the formation and to release carbon dioxide concomitantly whereby a beneficial effect due to the mutual miscibility of carbon dioxide in the fluid phases is realized as a reduction in viscosity and retentive capillary forces, while another beneficial effect is realized in the form of increased formation energy, due to the pressure generated by the released carbon dioxide.

An advantage resulting from the employment of the method of this invention in acidizing fluid-bearing formations is that the post-precipitation of dissolved carbonates is prevented or materially decreased. Such post-precipitation occurs because of the nature of the dissolution reaction:

Ca Hoop, CaCo H2O co, 7.

When pressure is released so that spent reaction products from the acidization process can be removed, carbon dioxide gas can break out of solution, causing post-precipitation of calcium carbonate. Such postprecipitation occurring within the formation matrix near the bore hole can decrease permeability by plugging the formation capillaries, particularly those near the well bore, and result in a lower production rate. Furthermore, such post-precipitation can occur in the tubing or annulus of the well itself and manifest itself as mineral seale, reducing their diameter(s) and resulting in a lower production rate.

The phosphate ester useful in preparing the novel composition of this invention is a phosphate ester having the general formula wherein R is either hydrogen or a group in which n is l to 3, m is equal to 2 or 3, and R is a hydrogen atom not more than m minus 1 times, or a mixture of said esters, said ester having a molecular weight in the range of from about 750 to about 1,730.

The phosphate esters in the composition of the present invention can be prepared in a known manner such as for example by ethoxylation of tridecanol and subsequent phosphorylation of the ethoxylated tridecanol.

The corresponding salts of the ester can be prepared by neutralization of the ester with caustic soda or potassium or ammonium hydroxide.

A preferred ester is an ester having the structural formula having a molecular weight of 754 and the empirical formula c n o r or a mixture of said ester with an ester having the structural formula v o 0 on H H CmIIuO(C II O) C;II [OP] -(OCzII4)3O-I OII OCQII4-(OCQII|)ZOC13IIQ7 which has the empirical formula The admixture of esters may contain from about 30 to percent by weight of the C H O P ester and about 70 to 30 percent by weight of the C H O P. ester. A preferred range is about 40 to 60 percent of the C ester and about 60 to 40 percent of the C ester.

Preferably, the aqueous acid composition of this invention is one comprising an aqueous solution which may or may not include brine, and about 3 to about 30 percent preferably 5-28 percent, by weight of a nonoxidizing mineral acid, and which contains dissolved therein between about 0.1 to about 2.5 percent by weight of the phosphate ester preferably 0.3 to 1.5 percent based on the total solution weight.

Generally, the aqueous solution will contain an inhibitor to prevent or greatly reduce the corrosive attack of the acid on metal. Any ofa wide variety of compounds known in the art and employed for this purpose can be used, e.g., certain compounds of arsenic, nitrogen or sulfur as described by Grebe et al in U.S. Pat. No. 1,877,504 or a rosin amine type as described in US. Pat. No. 2,758,970. The amount of the inhibitor utilized is not highly critical and it may be varied widely. Usually this amount is defined as a small but effective amount, e.g., from 0.02 percent to about 2.0 percent by weight.

In carrying out the method of this invention a solution of from about 3 to about 30 percent by weight of the non-oxidizing mineral acid dissolved in water is first prepared. An inhibitor to prevent corrosion of acid on the metal equipment associated with the well is usually added with mixing in the next step. The phosphate ester in an amount within the stated concentration range is then admixed with the aqueous acid solution. The thusprepared composition is forced, usually via a suitable pumping system, down the well bore and into contact with the production equipment and formation to be treated. As those skilled in the art will readily understand, the pressure employed is determined by the nature of the formation, viscosity of the fluid, and other operating variables. The acidization method of this invention may be carried out at a pressure sufficient merely to penetrate the formation or it may be of sufficient magnitude to overcome the weight of the overburden and create fractures in the formation. Propping agents, to prop open the fractures as created, for example 20 to 60 mesh sand, in accordance with known fracturing procedure, may be employed, in admixture with the aqueous acidic solution containing the cited phosphate ester. Generally, it is advisable to allow the aqueous acid solution to remain in contact with the formation until the acid therein has been substantially depleted by reaction with the acid-soluble components of the formation and the deposited scale. After this, the substantially spent treating solution is reversed out of the well, i.e., it is allowed to flow back out or to be pumped out of the formation. Further, as those skilled in the art will understand, the concentration of the phosphate ester and acid component should be chosen to provide an acidizing fluid of the desired rheological properties.

In the method of this invention, the phosphate ester component of the composition provides means whereby calcium ions having tendencies to precipitate as CaCO from a supersaturated solution of CaCO or Ca(l-lCO that is produced by the reaction of the acid component with the formation, or previously deposited acid-soluble mineral scale combined with the phosphoric acid moiety to form a highly stable complex therewith so that solid calcium carbonate does not precipitate from the spent treating solution. This binding up of the aforementioned calcium ions from weakly ionizable compounds permits the formed calciumphosphoric acid ester complex to remain dissolved in the treating solution and pass through pores and production equipment.

Further, the phosphate ester component of the composition provides means whereby the nucleation and growth of the solid itself is thwarted, so that solid calcium carbonate does not precipitate from the spent treating solution.

Finally, the phosphate ester component of the composition provides means whereby continuous protection against post-precipitation of CaCO is obtained for a considerable period of time subsequent to treatment due to continuous slow desorption of the component from the formation surfaces. In contrast, use of surfactants having merely dispersant and suspending properties and not possessing the capability of molecularly binding up these produced calcium ions or thwarting the nucleation and growth of the solid CaCO will permit deposition of calcium carbonate to occur from such treating solution with the likelihood of plugging the formation passageways and production equipment during subsequent recovery of desirable formation hydrocarbons therethrough.

Following is a description by way of example of the method of the invention.

EXAMPLE I A producing well would be treated in the following manner.

A treating mixture is prepared by mixing about 10 barrels of salt water containing about 2.6 percent sodium chloride and 12 barrels of 15 percent by weight aqueous hydrochloric acid containing 0.5 barrel of a mixture of l9.ll percent by weight of the phosphate ester sold under the trade name Phosphonol T, 4.98 percent by weight of 50 strength caustic, and 75.9] percent by weight of water. The ester employed is a mixture of esters of polyphosphonic acids containing polyethoxy linkages partially esterified with alkoxypolyethoxyethanols. Such mixed esters on typical analysis analyzed 51.9 percent carbon, 9.7 percent hydrogen, 8.8 percent phosphorous and 29.6 oxygen (by difference). The apparent empirical formula is C H P30"; and the mixture has acalculated molecular weight of 965. The neutralization number was found to be 253. NMR analysis showed the mixture to comprise highly branched alkylated ethoxylated acid phosphate containing a 5.1 ratio of ortho and pyrophosphates. The ratio of ethylene oxide radical (-CH CH l3O- tridecyl radical (C H and phosphate ester radical (CH -O-P=O) are 3:] :1, respectively.

The final treating mixture of 22 barrels of treating solution containing 34.3 pounds of Phosphonol T, (0.39 percent), 648.] pounds of HCl (7.48 percent), the balance salt water, with a little caustic is squeezed into the formation at a rate of about one-half BPM at 450 psig. The well is then returned to production.

EXAMPLE [I A treating mixture is prepared from 10 barrels of salt water (2.6 percent sodium chloride), and 12 barrels of 15 percent by weight aqueous hydrochloric acid solution containing 1 barrel of the same mixture and the same phosphate ester of Example I. The treating mixture composed of 68.4 pounds of Phosphonol T (0.787 percent), 621 pounds of HC] (7.14 percent), the balance salt water containing a little caustic, is injected into the producing formation in a manner approximating that used in Example I. Thereafter 20 barrels of water are used to overflush the treated formation by injection down the tubing,'followed by injection of 10 barrels of water down the casing. The well is then returned to production.

EXAMPLE Ill The treating mixture of Example II is injected into the producing formation and an overflush of 10 barrels of water is used to force the treating mixture into the formation by injection down the tubing. The well is then returned to production.

The method of the present invention is effective in stimulating and/or restoring the production of petroleum fluids from underground formations whose deliverability has been diminished or destroyed by post-precipitation of dissolved salts within the capillary openings of the formation subsequent to acidization and by deposition of acid-soluble mineral scale.

Furthermore, the ability of the disclosed phosphate ester to adsorb on the formation face during the cited acidization process and showly desorb as the well is produced provides continuous protection against recurrence of these two problems which lessen production.

Obviously, many modifications and variations of the invention as hereinabove set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

lclaim. I

l. A method of increasing the production of fluids from a subterranean fluid-bearing formation having present therein acid-soluble components comprising injecting down the well bore penetrating said formation and injecting therefrom into said formation under a pressure greater than the formation pressure, an aqueous solution containing from about 3 to 30 percent by weight of a mineral acid having dissolved therein from about 0.1 to 2.5 percent by weight of a phosphate ester having the general formula wherein R is either hydrogen or a C H O(C H C l-l group in which n is l to 3, m is equal to 2 or 3 and R is a hydrogen atom not more than m minus 1 times, A is hydrogen or an alkali metal, or a mixture of said esters, said ester having a molecular weight in the range of from about 750 to about 1,730, maintaining said solution in contact with the formation and production equipment for a time sufficient for the acid to chemically react with the acid-soluble components of the formation to etch passageways therethrough thereby increasing substantially the flow capacity of the said subterranean formation.

2. Method as claimed in claim 1 wherein said ester has the formula 3. Method as claimed in claim 1 wherein said ester has the formula 0.3 to 1.5 percent by weight.

5. Method as claimed in claim 1 wherein said ester is present in the form of the sodium salt.

6. Method as claimed in claim 1 wherein said acid is hydrochloric acid.

7. Method as claimed in claim 1 wherein the said acid is sulfuric acid.

8. Method as claimed in claim 1 wherein the said ester is dissolved in about 5 to about 20 percent by weight solution of the mineral acid.

9. Method as claimed in claim 1 wherein the said acid is about 5 to about 28 percent by weight hydrochloric acid and the said ester is present in an amount of from about 0.3 to about 1.5 percent by weight, based on the total weight of the composition.

10. Method as claimed in claim 1 wherein the said formation is a hydrocarbon-bearing formation.

11. Method as claimed in claim 1 wherein the said aqueous solution is injected down the well bore penetrating said formation under a pressure greater than the formation pressure and sufficient to create fractures in the formation.

12. Method as claimed in claim 1 wherein the said aqueous solution is injected down the well bore penetrating said formation under a pressure greater than the formation pressure but less than the pressure required to create fractures in the formation.

13. Method as claimed in claim 1 wherein said ester comprises a mixture of about 30-70 percent by weight of the phosphate ester having the formula:

and from about -30 percent by weight of the phosphate ester having the formula: 

1. A method of increasing the production of fluids from a subterranean fluid-bearing formation having present therein acid-soluble components comprising injecting down the well bore penetrating said formation and injecting therefrom into said formation under a pressure greater than the formation pressure, an aqueous solution containing from about 3 to 30 percent by weight of a mineral acid having dissolved therein from about 0.1 to 2.5 percent by weight of a phosphate ester having the general formula wherein R is either hydrogen or a C13H27O-(C2H40)n 1-C2H4 group in which n is 1 to 3, m is equal to 2 or 3 and R is a hydrogen atom not more than m minus 1 times, A is hydrogen or an alkali metal, or a mixture of said esters, said ester having a molecular weight in the range of from about 750 to about 1,730, maintaining said solution in contact with the formation and production equipment for a time sufficient for the acid to chemically react with the acid-soluble components of the formation to etch passageways therethrough thereby increasing substantially the flow capacity of the said subterranean formation.
 2. Method as claimed in claim 1 wherein said ester has the formula
 3. Method as claimed in claim 1 wherein said ester has the formula
 4. Method as claimed in claim 1 wherein said ester is present in the composition in an amount of from about 0.3 to 1.5 percent by weight.
 5. Method as claimed in claim 1 wherein said ester is present in the form of the sodium salt.
 6. Method as claimed in claim 1 wherein said acid is hydrochloric acid.
 7. Method as claimed in claim 1 wherein the said acid is sulfuric acid.
 8. Method as claimed in claim 1 wherein the said ester is dissolved in about 5 to about 20 percent by weight solution of the mineral acid.
 9. Method as claimed in claim 1 wherein the said acid is about 5 to about 28 percent by weight hydrochloric acid and the said ester is present in an amount of from about 0.3 to about 1.5 percent by weight, based on the total weight of the composition.
 10. Method as claimed in claim 1 wherein the said formation is a hydrocarbon-bearing formation.
 11. Method as claimed in claim 1 wherein the said aqueous solution is injected down the well bore penetrating said formation under a pressure greater than the formation pressure and sufficient to create fractures in the formation.
 12. Method as claimed in claim 1 wherein the said aqueous solution is injected down the well bore penetrating said formation under A pressure greater than the formation pressure but less than the pressure required to create fractures in the formation. 